In recent years, the number of full-color images is increasing in the field of electrophotography. In full-color images, an image is formed of a larger number of pixels compared to text images. Therefore, images have a tendency to have more regions of so-called solid image. Since a larger amount of toner passes through the fixing apparatus while fixing an image having such solid regions, generally silicone oil has been utilized to prevent offset when a fixing apparatus having a contact member is used. Although offset is reduced by employing silicone oil, silicone oil may remain on the image surface to cause glare or difficulty in additional writing on the image. Further, when silicone oil remains unevenly on the image surface, the image quality may be deteriorated due to the unevenness of the image.
To overcome this problem, a release agent, typically a wax, is added to the toner making silicone oil unnecessary when the image is fixed. Namely, an oil-less fixing method has been employed these days. However, even in this method, unevenness in gloss tends to occur due to the wax incorporated in the toner, resulting in degradation of image quality. This becomes problematic when using a fixing apparatus having a transport device which incorporates a contact member.
However, since a transport device employing a contact member is a simple and efficient transport means, it is rather difficult to be replaced with other methods. Accordingly, at present, a method to provide stable images using a full-color image formation method has not been fully established. On the other hand, with respect to a low-temperature fixing toner, a lower melting point resin or a lower softening point resin used for the low-temperature fixing toner often causes problems. However, disclosed have been methods to improve the low-temperature fixing toner in terms of selection of a wax used as a release agent (for example, refer to Patent Document 1).
Further, solution of the above-described problem has become more important for a so-called polymerized toner than for a pulverized toner, because, even for the polymerized toner which has recently been widely employed, oil-less fixing is becoming a main current as a fixing method, since addition of a release agent in the production process is easier for the polymerized toner. In the polymerized toner, a resin having a polar group is used for the reason of the production method. Accordingly, improvement in stability of electrostatic chargeability of the toner is desired, since the polar group has a hygroscopic nature.
Also, in view of a desire to conservation of resources and energy, a fixing process, which consumes the largest energy among electrophotographic processes, is expected to carry out at lower temperature and in a simple operation. In this point of view, the above-described oil-less fixing method and the transport method using a transport device having a contact member are advantageous for a low temperature and simple operation. Improvement in electrostatic chargeability, avoidance of image unevenness as well as releasability of the toner is expected.